This commit changes the the reference counting behavior, going
from add/sub with SEQ_CST/SEQ_CST to add/sub with
RELAXED/RELEASE+ACQUIRE.

This should make for more efficent clones and frees.

This method is inspired by the reference counter implementation in
the boost library.

https://www.boost.org/doc/libs/1_57_0/doc/html/atomic/usage_examples.html